1. Field of the Invention (Technical Field)
The present invention relates to electroslag remelting (xe2x80x9cESRxe2x80x9d) electrode immersion depth control systems and methods.
2. Background Art
As shown in FIG. 1, ESR furnaces 10 have been utilized for over 40 years to refine metals and produce fully dense homogeneous ingots 22. The remelting takes place by immersing a consumable metal electrode 14 into a molten slag bath 18 that is resistively heated through applied power 24 to a temperature above the melting point of the metal. The electrode gradually melts, forming metal droplets that fall through the slag and collect in a pool 20 under the slag. The molten pool is contained within a water-cooled mold 16 that has a slightly larger diameter than the electrode. As the electrode melts, it must be translated downward by an electrode drive 12 at a rate related to the fill ratio and the melt rate, as specified by the system controller 26. A complicating factor is that a small amount of slag solidifies on the surface of the mold, changing the amount of metal needed to fill the mold, and changing the thickness of the molten slag on top of the ingot.
To produce a high quality homogeneous ingot with good surface quality, the deviations in the processxe2x80x94specifically immersion depthxe2x80x94need to be minimized. To optimize process efficiency and surface quality, the immersion depth must be maintained at a constant level, as shallow as possible. However, the shallower the immersion depth, the more sensitive the process is to input or external variables, hence, the more difficult it is to control. If the immersion depth is allowed to get too shallow, gaps can form between portions of the electrode surface and the slag, leading to arcing, atmospheric exposure, and deleterious oxidizing reactions. Conversely, too large an immersion depth, or too much variability in depth, can lead to poor surface and metallurgical quality in the ingot.
Again, the ESR process is used to refine metal, remove inclusions, and produce ingots having a uniform solidification grain structure and good surface quality. The immersion depth is an important parameter to control since it has a major effect on the thermal conditions governing melting and solidification. Deviations in immersion depth will alter the thermal environment of the process, inducing changes in the melting process (rate, efficiency, configuration, droplet location and size) and on solidification parameters (rate, direction, molten metal flow). As a result, immersion depth fluctuations will result in changes to the ingot""s solidified grain structure, compositional homogeneity, and properties, and affect subsequent processing operations and final product quality.
Existing control methods drive the electrode in response to an error between the system voltage (which is related to immersion depth as described below) and a voltage set point. They utilize bi-directional electrode drive to oscillate around the set point, inherently resulting in constant fluctuation of the immersion depth. Shallower immersion depths have been shown to result in improved surface quality, hence improved process yields. The voltage of an ESR furnace system is most sensitive to immersion depth changes near the surface. Thus, it is more difficult to control electrode position at shallow immersions. As a result, existing control systems are only stable at a deeper immersion depth than would be associated with optimum surface quality.
No system currently exists to measure the depth directly, so it must be inferred from measured parameters of the process. At present, the ESR immersion depth is controlled in most systems by using the voltage and voltage swing, which is measure of the variation in the voltage. These methods are referred to as swing controllers.
The voltage is used because ESR furnaces primarily operate with a constant current power supply. At a simplified level, the slag can be viewed as a resistor, so the voltage is given by Ohm""s Law:
V=I[d/(Ak)]
where V is the voltage, I is the current, and the resistance of the slag is approximated by the expression in the brackets where d is the distance between the electrode and the molten metal pool, A is the area of the electrode in contact with the slag, and k is the slag conductivity. However, there are numerous simplifications inherent in this treatment, so voltage is only a rough indicator of the electrode immersion. Additionally, the slag thermal environment and chemistry will change over the course of a melt, hence its conductivity is not constant. The amount of molten slag will also change during a melt due to slag plating out on the cold crucible walls, further altering the above relationship.
Consequently, while voltage is an effective immediate indicator of relative electrode position with respect to the surface of the slag, voltage alone has not been adequate to indicate or maintain a constant average immersion depth over time. Voltage swing cannot be directly related to the immersion depth via an equation such as the one presented above, nor can it be used as an instantaneous indicator. On the other hand, voltage swing is less sensitive to the factors that can change during the course of a melt. Regardless of slag amount, conditions, or properties, the isopotential lines within the slag will be compressed near the surface of the slag. As a result, increases in voltage swing can be reliably, but not quantitatively, related to a reduction in immersion depth.
Existing control systems utilize changes in voltage swing to adjust the voltage set point in response to changing process conditions. The basic method shown in FIG. 2 shows a schematic of an existing control system. Over the short term, the drive speed is determined by multiplying the voltage error (Vrmsxe2x88x92Vsp) by a proportionality constant, Ke. This can be expressed by the equation: Drive Speed=Ke(Vrmsxe2x88x92Vsp) where Vrms is the system voltage applied to the electrode and Vsp is the voltage set point, a voltage indicative of desired electrode immersion depth. In the long term, the voltage swing is measured over a period of time and compared to a voltage swing set point. If the measured voltage swing is greater than the voltage swing set point, the immersion is taken to be too small, and the voltage set point is decreased. Conversely, if the measured voltage swing is smaller than the set point, the immersion depth is assumed to be too large, and the voltage set point is increased.
A more recently developed ESR control system was described in U.S. Pat. No. 5,737,355, to Damkroger, titled xe2x80x9cDirectly Induced Swing for Closed Loop Control of Electroslag Remelting Furnacexe2x80x9d. In this system, the electrode drive is the combination of a set unidirectional motion and a superimposed periodic fluctuation. This system then superimposes a periodic fluctuation of known amplitude (rather than electrode motion in response to a voltage error) to provide electrode motion relative to the isopotential lines in the slag, and thus generate the voltage swing signal. In the long term, positive deviations of voltage swing from the set point indicate too shallow immersion, and are used to increase the basic unidirectional drive speed. Negative deviations are used to do the opposite.
This directly induced swing system eliminated the confounding effect of the system""s own drive response on voltage swing. However, it incorporates no short-term response to an error, which limits its ability to operate very near the slag surface. Later modifications of the directly induced swing sought to address this shortcoming by incorporating a voltage error response as was used in the original swing controllers. The average is usually a long term average of the drive speed. Over the long term, the voltage swing is measured and deviations from its set point are used to adjust the voltage set point, usually with a linear gain factor. To some extent these modifications mitigated the problem but the immersion depth was still too deep.
The pattern of periodic fluctuations in the impedance of the ESR process, referred to in this document as impedance spikes, are a phenomenon discovered in 1993 by Sandia National Laboratory researchers in the Specialty Metals Processing Consortium. These spikes are most conveniently calculated from measured electrode voltage and current and represent inherent fluctuations in the system""s characteristic impedance that are not associated with electrode motion relative to the slag. Instead, they result from the variation in the melting of the electrode and the rapid change in slag/electrode contact area at shallow immersion depths. Initially the spike is the result of increased electrode melting, leading to a change in the immersion depth. Then contact resistance becomes a factor. The magnitude of contact resistance is considerably greater than the slag resistance, so changes in contact area between the electrode and the slag result in major changes in system impedance. These changes increase dramatically at shallow immersion depths. At shallow depths the impedance spikes are by far the most dominant forces in generating the natural variation of the voltage in ESR furnaces.
The existence of impedance spikes was unknown prior to 1993 because they were masked by the electrode drive response of voltage swing controllers and by melting at deeper immersion depths. Current was held constant, so the impedance spikes were manifested as voltage increases. The system""s response was to drive the electrode downward, damping the spike. Because of the magnitude of the spikes, large responses were required, resulting in severe variations in immersion depth and process conditions and also deeper immersion depths were used to reduce the magnitude of the variation. Evaluation of past data shows that impedance spikes have long been a factor affecting ESR drive response.
In 1993, the application of directly induced swing controllers revealed the existence of the impedance spikes. The spikes were not damped by any short term response to voltage errors, and although the electrode was being driven down at the average speed required to match the fill ratio for the melt rate, the spikes appeared as rapid increases in system voltage.
FIG. 3 shows the voltage rise in response to two impedance spikes at a shallow immersion depth. The large spike at 25 seconds is believed to represent a larger increase in contact resistance than the smaller spike at 50 seconds. The rapid deceases in the voltage at the end of the spikes is a result of the controller""s voltage error response. In order to keep the electrode from arcing (which would occur if the electrode lost contact with the slag pool), the controller responds to the dramatic overvoltages by driving the electrode down at a rapid rate.
Existing control systems use very high electrode drive speeds to overcome the impedance/voltage spikes near the slag surface. The gain of their voltage response is necessarily much higher than needed to respond to non-spike fluctuations, so the system overreacts much of the time and drives the electrode excessively up and down in the slag. Part of that is by design, because the controller needs to generate the voltage swing. Also, because the spikes are asymmetric but current systems respond symmetrically to the voltage error, the systems tend to drive the electrode up in the slag too fast, leading to instabilities. The resulting average immersion depths are too deep, and subject to both short and long term fluctuations.
An additional result is that the spikes dominate the voltage swing calculations. Because of the rapid change in voltage close to the surface, the swing calculations will vary significantly at shallow immersions, leading to oscillations in the voltage set point. These changes cause spurious variations in the immersion depth.
A primary object of the present invention is providing for more stable ESR furnace control.
A primary advantage of the present invention is that it permits shallower, more stable electrode immersion depths than previous systems, resulting in higher quality ingots.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a method of controlling the electrode drive speed on an electroslag remelting furnace including the steps of: initially driving the electrode at a nominal rate consistent with the melting rate and geometry; adjusting the electrode drive speed by an amount proportional to the difference between a measured metric of the proximity of the electrode tip to the surface of the slag pool and a set point, and additionally adjusting electrode drive speed by a second amount if a measured metric of the proximity of the electrode tip to the surface of the slag differs from a set point by a predetermined amount; wherein the second amount is greater than the first amount. Additionally it comprises a means for periodically adjusting the first set point by monitoring characteristics related to the impedance spikes and determining if the overall immersion depth needs to be changed.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.